Method of controlling the intensity of an electron beam

ABSTRACT

A method for controlling the cross-sectional area and intensity of an electron beam produced in an electron accelerator. A first constant electric field is applied to the beam causing dispersal of the beam in a first plane and concentration of the beam in a second plane disposed perpendicular to the first plane. Subsequently, a second constant electric field is applied to the beam causing beam concentration in the first plane and beam dispersal in the second plane. The fields are applied to the beam between the origin of the beam and the accelerator window and provide that the cross-sectional area of the beam greatly is enlarged at the point of intersection of the beam and the window relative to its original dimensions.

United States Patent [72] Inventor Zia l-Iashmi Westland, Mich.

[2i Appl. No. 888,279

[22] Filed Dec. 29, 1969 [45] Patented Nov. 16, 1971 [73] Assignee Ford Motor Company Dearborn, Mlch.

[54] METHOD OF CONTROLLING THE INTENSITY OF AN ELECTRON BEAM 7 Claims, 3 Drawing Figs.

[52] US. Cl 315/31, 250/495 [5 l J Int. Cl H01j 29/56 [50] Field of Search 250/495 C; 313/74; 315/31 [56] References Cited UNITED STATES PATENTS 2,680,815 6/1954 Burrill 313/74 X 3,270,243 8/1966 Kerst 313/74 X 2,866,902 12/1958 Nygard.... 250/495 C 3,028,491 4/1962 Schleich 250/495 Primary Examiner-Rodney D. Bennett, .lr. Assistant Examiner-J. M. Potenza Attorneys-John R. Faulkner and E. Dennis O'Connor ABSTRACT: A method for controlling the cross-sectional area and intensity of an electron beam produced in an electron accelerator. A first constant electric field is applied to the beam causing dispersal of the beam in a first plane and concentration of the beam in a second plane disposed perpendicular to the first plane. Subsequently, a second constant electric field is applied to the beam causing beam concentration in the first plane and beam dispersal in the second plane. The fields are applied to the beam between the origin of the beam and the accelerator window and provide that the cross-sectional area of the beam greatly is enlarged at the point of intersection of the beam and the window relative to its original dimensions.

PATENTEnunv 1s ISTI 3.621.327

' sum 1 [IF 2 METHOD OF CONTROLLING THE INTENSITY OF AN ELECTRON BEAM BACKGROUND OF THE INVENTION Electron-beam generators or electron accelerators having accelerating voltages in the order of several million volts conventionally include a long insulating container. This container defines an evacuated chamber through which electrons are accelerated to form a beam by means of a large potential difference. existing between an electron gun, including a hot cathode emitter at one end of the chamber, and an anode at the other end of the chamber. The anode includes an electronpenneable window through which the beam passes from the chamber onto a substance being irradiated.

Conventional materials for the formation of electronpenneable windows are thin foils that allow the passage of the electron beam and can be supported in an opening in a container wall so that the vacuum within the chamber is maintained. As the electron beam passes through the metal window, the electrons are scattered to some extent with a consequent heating of the window. It a portion of the metal window is heated beyond a tolerable level as the beam is scanned along the window, this overheating causes oxidation and weakening of the metal and resultant punctures through the metal. The necessary vacuum seal otherwise provided by the window obviously is destroyed upon the formation of punctures.

In order to avoid such overheating of the window, the electron intensity of the beam must be controlled rigidly. Some prior art procedures for controlling beam intensity have resulted in beam generator operation below the potential output of this machine and are unsatisfactory in this respect.

Another well known procedure for preventing the window overheating due to excessive beam intensity is known as beam scanning. According to this procedure, the beam is scanned over the surface of an elongated window thereby preventing local overheating at any point on the window. It is well-known that beam scanning requires the application of an electric field (either electrostatic or magnetic) that is time varying in order to achieve the desired variance in the beam path.

Such scanning methods have certain inherent features that may be undesirable. An appropriate cycle for the varying fields must be established to achieve the desired beam scanning movement. Also, this field cycle must be synchronized with the rate of movement of the target material being irradiated, as such material conventionally is moved continuously past the accelerator window as on a conveyor line to promote maximum utilization of the available radiation energy. The field varying and synchronizing circuits and the hardware necessary for scanning methods are relatively complex, difficult to maintain and subject to break down.

It is an object of this invention to provide a method for controlling the electron beam produced in an accelerator such that the intensity of the beam is reduced at the accelerator window whereby local overheating of the window does not occur. To accomplish this end, this method contemplates defocusing of the beam between its point of origin and the window by applying to the beam unvarying electric fields, the formation of which requires only relatively simple and reliable apparatus. Also, by reducing the intensity of the beam and thus increasing its cross sectional area, this method provides for a larger feasible beam target area.

SUMMARY OF THE INVENTION The method of this invention is directed to controlling the cross-sectional area of an electron beam produced in an evacuated container and directed along a straight line toward an electron permeable window in the wall of the container. The method comprises steps of both concentrating the beam in a first plane and dispersing the beam in a second plane at a first location. The first and second planes are perpendicular to one another with the intersection of the planes comprising the straight line along which the beam originally is directed. At a second location between the first location and the accelerator window the beam is dispersed in the first plane and concentrated in the second plane. These steps accomplish an increase in the cross-sectional area of the beam and a corresponding decrease in the intensity of the beam at the accelerator window. The first and second locations are spaced apart and the concentration of the beam at the first location causes a focusing of the beam at a focal point located between the first and second locations.

DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view illustrating schematically an electron accelerator that may be used to practice the method of this invention;

FIG. 2 is a schematic representation in a first plane of the beam envelope produced in the accelerator of FIG. 1 and controlled according to the method of this invention;

FIG. 3 is a view similar to FIG. 2 but illustrating the beam envelope in a second plane that is perpendicular to the first plane of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION Referring now in detail to the drawings and in particular to FIG. 1 thereof, the numeral 10 denotes generally an accelerator useful in the practice of the method of this invention. This accelerator includes an elongate container 12 defining an internal chamber 14. Proximate one end wall 16 of container 12 is a hot cathode emitter 18. Remote from end wall 16, container 12 includes a tapered portion 20 terminating in an end wall 22. The end wall 22 has formed therethrough an aperture in which is mounted an electron permeable window 24.

As is conventional, window 24 is constructed of electronbeam-permeable material such as thin metal foil. The window substantially is square in shape and the window center is the point of intersection of a pair of perpendicular axes designated by the letters X and Y and illustrated to aid in the clarity of this description. Window 24 is mounted in the aperture formed in container end wall 22 such that it provide an airtight seal of this aperture so that a vacuum may be maintained within chamber 14. Window 24 is included in the anode structure of the accelerator. A great potential difference exists between this anode structure and cathode 18 so that electrons emitted by the cathode form an electron beam directed at the window 24.

Control of the cross-sectional area, and thus the intensity of the electron beam originating at cathode 18 is made possible by the presence of electromagnetic coils positioned about the container 12. A first pair of coils 26 and 28 are positioned diametrically across the container 12 and may be considered the X-axis coils since they are positioned in the plane containing the X-axis. (The Y-axis intersects and is perpendicular to this plane.) Downstream from coils 26 and 28 are a second pair of coils 30 and 32. Each of the coils 30 and 32 is positioned about the outer periphery of the container from each of the coils 26 and 28. The coils 30 and 32 may be termed the Y-axis coils.

Coils 26 and 28 are connected to a suitable source of electrical energy such that these coils give rise to a steady state or unvarying field through that portion of chamber 14 that is located between these coils. In a similar manner, coils 30 and 32 are connected electrically to a source of energy such that they give rise to a steady state or unvarying field located therebetween. The manner in which the electric fields created by the coils 26, 28, 30 and 32 are utilized to control the electron beam of the accelerator may be seen by reference to FIGS. 2 and 3 of the drawings.

In FIGS. 2 and 3, the electron beam envelope is designated by the numeral 34. The direction of travel of the beam is identified by the arrow 36. It may be seen that the beam originates at the cathode 18 as a relatively thin, straight line flow of electrons. The cross sectional area of the beam at this time is quite small since beam diameters of the order of a few millimeters are common. This portion of the beam is designated by the reference numeral 38.

It may be appreciated that the beam, downstream from cathode l8, enters the electromagnetic field generated by the X-coils 26 and 28 and represented schematically by the square 40. The portion of the beam within field 40 is identified by the reference numeral 42. From FIG. 2, it may be seen that this field causes a divergence or defocusing of the beam portion 42 in the so-called X-plane, that is, the plane of the drawing of FIG. 2. The X-plane divergence given to the beam portion 42 as it passes through the field 40 continues, of course, after the beam has left the field 40. The portion of the beam immediately downstream from field 40 is identified by the reference numeral 44.

In the Y-plane, that is, the plane of the drawing of FIG. 3, the beam portion 42 within field 40 is caused to converge or be focused. lt readily may be appreciated that a homogeneous steady state electromagnetic field that causes a beam to be defocused in a first plane (the X-plane will simultaneously cause a beam to be focused in a second plane (the Y-plane) that is perpendicular to the first plane. This is the effect that field 40 has upon beam 34.

The portion of the beam designated by the numeral 44, that is, that portion of the beam immediately downstream from the field 40, diverges in the X-plane of FIG. 2 since no electric field is acting thereupon. in the Y-plane (FIG. 3), beam portion 44 is converging as it leaves the field 40 as at 440 It thus becomes focused in the Y-plane at a focal point 46. Naturally, downstream of the focal point 46, beam portion 44 begins to diverge as at 44b.

The electric field caused by the Y-coils 30 and 32 is represented schematically by the square 48 in FIGS. 2 and 3. it may be seen that the portion of the beam passing through this field is designated by the numeral 50. In the X-plane, beam portion 50 is acted upon by field 48 and the divergence that continues along beam portion 44 is arrested and a slight convergence of the beam at beam portion 50 occurs. This action of field 48 prevents too great a lessening of the beam intensity and creates a controlled X-plane dimension and beam intensity at the window 24. It may be seen that this slight convergence caused by field 48 continues downstream of the field 48 whereat the beam portion is designated by the numeral 52.

While field 48 causes a convergence of the beam in the X- plane, it may be seen from FIG. 3 that a divergence of the beam is caused by the field 48 in the Y-plane. The slight beam divergence of beam portion 44b is increased in beam portion 50. This divergence continues of course downstream of field 48 along beam portion 52 until the beam intersects the window 24.

it may be seen from a comparison of FIGS. 2 and 3 that, at window 24, the beam dimensions along the X- and Y-axes approximately are the same and that these dimensions are much greater than corresponding dimensions of the beam prior to the entry of the beam into the fields created by the electromagnetic coils. Since the beam cross-sectional area at the window is much greater than at the point or origin, the beam intensity at the window accordingly is decreased relative to its original intensity. The beam thus constantly may be applied to window 24 without the danger of overheating. The extent to which a particular electron beam is defocused and its intensity is decreased is dependent of course, on the particular apparatus involved and easily may be determined empirically. it thus may be seen that the method of this invention allows an electron beam to be controlled so that there is no need for scanning of the beam along the accelerator window or for operation of the accelerator at less than full capacity.

I claim:

1. A method for effecting the efiicient distribution of available ionizing energy from an electron beam produced in an evacuated container and directed along a first axis toward an electron-permeable window in a wall of said container, said method comprising the steps of: app] ing a first field to said beam, said field causing said beam to concentrated along a second axis that is perpendicular to said first axis and dispersed along a third axis that is perpendicular to both said first and second axes, and applying a second field to said beam, said second field being spaced along said first axis from said first field, causing said beam to be dispersed along said second axis and concentrated along said third axis.

2. A method according to claim 1, wherein said first and second fields are electrostatic.

3. A method according to claim 2, wherein said first and second fields are applied to said beam at locations between the point or origin of said beam and said window.

4. A method according to claim 2, wherein said first and second fields are constant in magnitude.

5. A method according to claim 2, wherein the concentration of said beam due to said first field causes a focusing of said beam along said second axis, the focal point being located between said first and second fields.

6. A method for controlling the cross-sectional area of an electron beam produced in an evacuated container and directed along a straight line toward an electron permeable window in a wall of said container, said method comprising the steps of: at a first location concentrating said beam in a first plane and dispersing said beam in a second plane, said first and second planes being perpendicular to one another and the intersection of said planes comprising said straight line, and at a second location between said first location and said window dispersing said beam in said first plane and concentrating said beam in said second plane, whereby the crosssectional area of said beam is greater at said window than at the point of origin of said beam.

7. The method of claim 6, wherein said first and second locations are spaced apart, and wherein the concentration of said beam at a focal point located between said first and second locations. 

1. A method for effecting the efficient distribution of available ionizing energy from an electron beam produced in an evacuated container and directed along a first axis toward an electron-permeable window in a wall of said container, said method comprising the steps of: applying a first field to said beam, said field causing said beam to be concentrated along a second axis that is perpendicular to said first axis and dispersed along a third axis that is perpendicular to both said first and second axes, and applying a second field to said beam, said second field being spaced along said first axis from said first field, causing said beam to be dispersed along said second axis and concentrated along said third axis.
 2. A method according to claim 1, wherein said first and second fields are electrostatic.
 3. A method according to claim 2, wherein said first and second fields are applied to said beam at locations between the point or origin of said beam and said window.
 4. A method according to claim 2, wherein said first and second fields are constant in magnitude.
 5. A method according to claim 2, wherein the concentration of said beam due to said first field causes a focusing of said beam along said second axis, the focal point being located between said first and second fields.
 6. A method for controlling the cross-sectional area of an electron beam produced in an evacuated container and directed along a straight line toward an electron permeable window in a wall of said container, said method comprising the steps of: at a first location concentrating said beam in a first plane and dispersing said beam in a second plane, said first and second planes being perpendicular to one another and the intersection of said planes comprising said straight line, and at a second location between said first location and said window dispersing said beam in said first plane and concentrating said beam in said second plane, whereby the cross-sectional area of said beam is greater at said window than at the point of origin of said beam.
 7. The method of claim 6, wherein said first and second locations are spaced apart, and wherein the concentration of said beam at a focal point located between said first and second locations. 